Stopping macro expansion - c

Question

I have a problem with macro expansion deferral. Here is an example:
#include <stdio.h>
#define CONST_ABC 15
#define CONST_5 7
#define ABC 5
#define PRINT(x) printf("CONST=%d\n", CONST_ ## x)
// The problematic macro
#define PRINT2(x) PRINT(x)
int main(int argc, char *argv[])
{
PRINT(ABC); // Prints 15 - OK
PRINT2(ABC); // Prints 7 - Not OK.
}
How to define PRINT2 macro so that it will use PRINT and result would be 15? I'm getting:
CONST=15
CONST=7
And want to get:
CONST=15
CONST=15

Answer 1

It requires you to have at least a C99 compiler, since C99 allows empty macro arguments. However some compilers may allow them as an extension, even in C89 mode. Here is the code:
#include <stdio.h>
#define CONST_ABC 15
#define CONST_5 7
#define ABC 5
#define PRINT(x) printf("CONST=%d\n", CONST_ ## x)
// The problematic macro
#define PRINT2(x, y) PRINT(x ## y)
int main(int argc, char *argv[])
{
PRINT(ABC); // Prints 15 - OK
PRINT2(ABC,); // Prints 7 - Not OK.
}
The second argument (i.e. the y) is empty, making it an empty preprocessing token. The ## operator prevents argument expansion, so the result of the concatenation is the same as x argument.
C11 6.10.3.1/p1 Argument substitution (emphasis mine):
After the arguments for the invocation of a function-like macro have
been identified, argument substitution takes place. A parameter in the
replacement list, unless preceded by a # or ## preprocessing token or
followed by a ## preprocessing token (see below), is replaced by the
corresponding argument after all macros contained therein have been
expanded. Before being substituted, each argument’s preprocessing
tokens are completely macro replaced as if they formed the rest of the
preprocessing file; no other preprocessing tokens are available.

Answer 2

The macro replacement, basically, proceeds as follows:
A token is found, which is a macro name
The arguments of the macro are collected
The arguments are substituted for the formal parameters in the body of the macro definition
Thus substituted parameters are completely macro replaced, not counting the rest the of input; stringification may be performed at this stage as well;
The token paste operators are performed
Thus substituted sequence is re-scanned, together with the rest of the input for further macro replacements
(plus some not well defined rules when a macro is forbidden for replacement)
The only way to prevent a macro argument from being macro-replaced in 4.
is for it to be followed or preceded by a token paste operator (##).
However, in 5. the paste operator has to perform the operation with
the argument under discussion and a special placemarker token. A placemarker token is inserted only for empty argument substitution.
Check this, it might give you an idea for your real code:
#define PRINT2(noreplace,x) PRINT(noreplace ## x)
PS. and yeah, "noreplace" is meant to be empty :)
PRINT2(,ABC)